In most filament cutters, the cutting filament or strip is stored on a spool or reel from which a cutting length extends. During use, wear on the cutting length shortens that length and it is then necessary to withdraw filament or strip from the spool to make up the cutting length to its working value.
This withdrawal of the filament or strip from the reel may be performed by the operator, either manually, for example by turning off the cutter, grasping the end of the filament and pulling it out of the housing or by operating a feeding mechanism while the motor is still running. However, it is preferable that the withdrawal of filament be effected without the intervention of the operator as this requires less skill from the operator, who does not need to decide when more line is required and also avoids the need to interrupt a cutting operation. In the case of a filament trimmer in which the filament is rotated by an electric motor, it has been proposed to sense the motor current electronically and when a predetermined decrease in current is detected (indicating a decrease in load and hence cutting length) filament is withdrawn from the spool to increase the cutting length. Such an arrangement is costly and each cutter requires individual setting of the electronic sensors to ensure correct operation.
It has also been proposed to control the withdrawal of filament from the spool by means responsive to centrifugal force acting on a member which permits withdrawal of filament from the spool when the sensed centrifugal force is of a predetermined value. Many proposals of this nature have been made, but it has been found that because the means responsive to centrifugal force responds to changes in motor speed when running, such systems are not suitable for mass production. The running speed of the motor cannot be sufficiently reliably controlled during manufacture, and the cutter thus requires individual setting and adjustment to maintain consistent operation. In an alternative system, the means responsive to centrifugal force, responds to changes in centrifugal force experienced on start-up and stopping of the cutter, when no withdrawal of filament is needed, thereby wasting filament. In more recent proposals, for example those of EP-A-171,533 (Emerson) and US-4 607 431 (Gay), the withdrawal or feed of filament is performed automatically by balancing the pull on the filament extending from the head into the cutting plane resulting from centrifugal force against the centrifugal force acting on a fixed or reference counterweight using a pivoted balance bar. This has the advantage of making the line feed independently of the speed of rotation, but in the Emerson system this has involved the line making frictional contact with the balance bar, impeding the freedom of the line to feed from the spool and in both the Emerson and the Gay systems there is a possibility that the balance bar will not return to a locking position after an increment of line has been fed thus allowing further line to feed. The operation of the device is thus made less reliable.